Methodology

Our forecasting model produces predictions for Democratic and Republican two-party vote share at the state and national level for every day in the election campaign. This section describes the method with which it arrives at those predictions.

Following the model description is a summary of how the method performed in the 2008, 2012 and 2016 presidential elections.

Structure

It is useful to think of the model as a combination of pre-election polls with other data about election outcomes, sometimes referred to as the “fundamentals” of the campaign. These data translate the president’s popularity and the economic environment into long-range forecasts of the final election outcome, made as early as March and updated throughout the election year. In the Bayesian sense, the model is using state and national polls to update these prior expectations of the political environment.

We are interested not only in the precise vote-share predictions from the model, but also the level of certainty we do(n’t) have in them. The model is thus made up of three different conceptual components:

  1. Prior predictions
  2. Polling averages
  3. Exploration of uncertainty

The first two are easier to explain than the latter. But before we get there, let’s address what data the model is ingesting.

Data

TK Prior: Head-to-heads + economic index throughout the election year Polls: Well, polls :)

Prior

TK

Poll averaging

TK

Uncertianty (or, how wrong could we be?)

TK


Model performance

Overall:

Select an election for more:

2008

Map

Final electoral college histogram

National and state polls and the electoral college over time

State vs national deltas over time

Model results vs polls vs the prior

Performance

outlet ev_wtd_brier unwtd_brier states_correct
economist (backtest) 0.0277172 0.0286135 49

Predictions for each state

2012

Map

Final electoral college histogram

National and state polls and the electoral college over time

State vs national deltas over time

Model results vs polls vs the prior

Performance

outlet ev_wtd_brier unwtd_brier states_correct
Linzer NA 0.0038000 NA
Wang/Ferguson NA 0.0076100 NA
Silver/538 NA 0.0091100 NA
Jackman/Pollster NA 0.0097100 NA
Desart/Holbrook NA 0.0160500 NA
economist (backtest) 0.0316938 0.0225292 51
Intrade NA 0.0281200 NA
Enten/Margin of Error NA 0.0507500 NA

Predictions for each state

2016

Map

Final electoral college histogram

National and state polls and the electoral college over time

State vs national deltas over time

Model results vs polls vs the prior

Performance

outlet ev_wtd_brier unwtd_brier states_correct
economist (backtest) 0.0820646 0.057329 47
538 polls-plus 0.0928000 0.066400 46
538 polls-only 0.0936000 0.067200 46
princeton 0.1169000 0.074400 47
nyt upshot 0.1208000 0.080100 46
kremp/slate 0.1210000 0.076600 46
pollsavvy 0.1219000 0.079400 46
predictwise markets 0.1272000 0.076700 46
predictwise overall 0.1276000 0.078300 46
desart and holbrook 0.1279000 0.082500 44
daily kos 0.1439000 0.086400 46
huffpost 0.1505000 0.089200 46

Predictions for each state